The invention relates to an error and status detection circuit for use in locaating a faulty pulse regenerator or cable section in a digital transmission system.
Digital transmission systems generally use pulse regenerators spaced along the transmission medium to regenerate each pulse after it has been attenuated somewhat by the medium. In general during each clock instant, the regenerators will regenerate a pulse for further transmission by comparing the level of the received pulse with a threshold level to decide what is the state of the pulse to be transmitted. A fully operative regenerator having an eye opening, or margin, in the range of 50 to 75 percent performs successfully at a very low error rate. A partially degraded regenerator having an eye opening in the range of 25 to 30 percent can regenerate data pulses marginally at a very low error rate. A malfunctioning regenerator having an eye opening of less than 20 percent, however, can cause a significant error rate.
Typical fault locating is accomplished by applying to the transmitting end of the digital system a trial stressing signal, or a pattern of digital signals including stressing. The status of each regenerator station in response to the trial stressing signal is communicated back to the transmitting terminal for analytical processing. Such stressing and analysis continues with increased stressing being used for subsequent trials until one of the regenerators makes significant errors and is identified.
In order to determine the location of a fault in a transmission line having all new or recently realigned and fully operative regenerators, except for one malfunctioning regenerator, a stressing signal of 40 percent probably would locate the malfunctioning regenerator or a faulty cable section without making a false indication that any other regenerator is faulty.
In a transmission line having many older partially degraded regenerators, except for one malfunctioning regenerator, a stress level of no more than 20 percent can cause one or more of the partially degraded regenerators to be mistaken for the actual faulty regenerator or cable section. Such a masking of the fault location is caused by errors generated by one of the degraded regenerators before the actual faulty regenerator is located.
Thus it can be understood that there is a problem in identifying a malfunctioning regenerator or faulty cable section without mistakenly identifying a degraded but functional regenerator instead of locating the actual fault. Identifying any regenerator, other than the actual fault location, can cause high mainenance costs because time and effort first is expended in servicing the marginally operational but apparently faulty regenerator station. After the marginal but apparently faulty regenerator is serviced, the actual malfunctioning regenerator or cable section still must be located and serviced.
A solution to this problem is to increase the sensitivity of the detection system for detecting the malfunctioning regenerator at a lower stress level that avoids a partially degraded but functional regenerator from masking the location of the actual malfunctioning regenerator or faulty cable section.